Can anyone explain this?

Question
1

I know it’s Friday, so perhaps my neurons are misfiring, but I’ve gazed at this for a few minutes, and I just can’t figure out how it’s possible.

Welcome to Scala 2.12.4 (OpenJDK 64-Bit Server VM, Java 1.8.0_191).
Type in expressions for evaluation. Or try :help.

scala> import java.security._
import java.security._

scala> Option(AlgorithmParameters.getInstance("AES"))
res0: Option[java.security.AlgorithmParameters] = Some(null)
1 Like
2

It’s not that magical AlgorithmParameters (Java Platform SE 8 )

public final String toString()
Returns a formatted string describing the parameters.
Overrides:
toString in class Object
Returns:
a formatted string describing the parameters, or null if this parameter object has not been initialized.

3

Wow, that’s pretty crazy, toString() returning null?!?

Good find, thanks!

4

Yep. Imperative programming at its finest. It even trips the REPL:

scala> java.security.AlgorithmParameters.getInstance("AES")
java.lang.NullPointerException
  at scala.runtime.ScalaRunTime$.replStringOf(ScalaRunTime.scala:268)
  at .$print$lzycompute(<console>:9)
  at .$print(<console>:6)
  at $print(<console>)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:498)
  at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:742)

The code where NPE is thrown:

  /** stringOf formatted for use in a repl result. */
  def replStringOf(arg: Any, maxElements: Int): String = {
    // here s is null, because arg.toString is null
    val s  = stringOf(arg, maxElements)
    val nl = if (s contains "\n") "\n" else ""

    nl + s + "\n"
  }
1 Like
5

This is a great puzzler.

Thanks to Som Snytt, the 2.13 REPL does a better job:

scala 2.13.0-RC3> class C { override def toString = null }
defined class C

scala 2.13.0-RC3> Option(new C)
res1: Option[C] = Some(null)

scala 2.13.0-RC3> new C
res2: C = null toString

see https://github.com/scala/scala/pull/6706. with bonus tpolecat!